1. Field of the Invention
The invention relates to methods and apparatus for determining, both uphole and downhole, the properties of oil. The invention more particularly relates to methods and apparatus for determining the precipitation onset pressure of certain asphaltenes. The invention has particular application to both oilfield exploration and production, although it is not limited thereto.
2. State of the Art
One of the problems encountered in crude oil production is asphaltene plugging of an oil well. Asphaltenes are components of crude oil that are often found in colloidal suspension in the formation fluid. If for any reason the colloidal suspension becomes unstable, the colloidal particles will precipitate, stick together and, especially in circumstances where the asphaltenes include resins, plug the well. Asphaltene precipitation during production causes severe problems. Plugging of tubing and surface facilities disrupts production and adds cost. Plugging of the formation itself is very difficult and expensive to reverse, especially for a deep water well.
Asphaltenes can precipitate from crude oils during production of the crude oil due to a drop in pressure. Crude oils which are somewhat compressible are particularly susceptible to this effect because the reduction in dielectric constant per unit volume which accompanies fluid expansion causes the asphaltene suspension to become unstable.
Asphaltenes are colloidally suspended in crude oils in micelles which are approximately 5 nm in diameter (See Asphaltenes, Fundamentals and Applications,xe2x80x9d E. Y. Sheu, O. C. Mullins, Eds., Plenum Pub. Co. New York, N.Y. 1995). With pressure reduction or addition of light hydrocarbons, the suspension can become unstable such that colloidal asphaltene particles stick together and flocculate or precipitate out of the solution.
The onset of asphaltene precipitation is difficult to predict, and when asphaltene plugging happens, it usually happens unexpectedly. Advance warning of asphaltene precipitation based on laboratory testing of formation fluid according to present techniques, while useful, is not optimally reliable.
Previously incorporated co-owned U.S. Ser. No. 09/395,141 to Mullins et al. discloses the use of the fluorescence-quenching properties of colloidally dispersed asphaltenes in determining the onset pressure of asphaltene precipitation. In particular, it was found that as asphaltenes precipitated out of the oil, the fluorescence of the oil increased. Thus, by changing the pressure on the oil sample, measuring the intensity of fluorescence at one or more wavelengths, and detecting a change either in intensity or in spectral shift of intensities across the spectrum of the fluorescence, the onset pressure of asphaltene precipitation could be found. It was also found that a downhole optical transmission measurement technique could be used to find the onset pressure, by finding a change in the total optical transmission of light through an optical cell.
While the methods of U.S. Ser. No. 09/395,141 are extremely useful, it has been determined by the inventors that the fluorescence-quenching technique is not as robust as might be desired, because only a small percentage of the asphaltenes present in the oil precipitate out of the oil at the onset pressure. Likewise, the optical transmission measurement technique is not as robust as might be desired because the change in total light transmission due to asphaltene precipitation is not specific. In addition, while the methods of U.S. Ser. No. 09/395,141 are useful in finding the asphaltene precipitation onset pressure, it appears that asphaltene precipitation does not in all cases lead to asphaltene plugging.
It is therefore an object of the invention to provide methods and apparatus for determining the precipitation onset pressure of sticky asphaltenes.
It is another object of the invention to provide robust methods for finding the precipitation onset pressure for asphaltenes of different particle sizes.
It is a further object of the invention to provide both uphole (laboratory) and downhole (borehole/wellbore) methods for finding the onset pressure of resin-containing asphaltenes which utilize optical measurements.
In accord with the objects of the invention which will be discussed in more detail below, the preferred embodiment of the method invention generally includes monitoring the optical density of an oil sample at a plurality of wavelengths over a plurality of different (typically decreasing) pressures, and using the optical density information to find the size of agglomerated asphaltene particles which are precipitating from the oil sample. Preferably, the optical density information used in finding the particle size is optical density information relating to the scattering of light due to the asphaltene particles only. Thus, according to the preferred embodiment of the invention, baseline optical density information of the oil sample at a high pressure is subtracted from optical density information obtained at test pressures at each wavelength of interest.
In accord with the invention, asphaltene precipitates having a diameter of approximately one micron or smaller are thought to be deficient in resins and are therefore unlikely to cause well-plugging problems. Thus, for purposes of determining precipitation onset pressures, the asphaltene particle size of interest is approximately one micron and larger. It is noted that since asphaltenes are insoluble in crude oil, it is resins which permit the asphaltenes to be suspended in the oil. Asphaltenes which have less resin attached to them are less stable, and are more likely to precipitate with smaller agglomeration sizes. Asphaltenes with more resins attached to them will tend to agglomerate to larger sizes during precipitation.
According to another aspect of the invention, additional optical density measurements are made as the pressure is increased on the sample which has already undergone precipitation, as it has been found that asphaltenes which do not have resins removed from them will reversibly re-suspend in the crude oil under certain circumstances. By making measurements in both decreasing and increasing pressure situations, and comparing the two, other optical scattering effects can be removed from the measurements, as only optical scattering from asphaltenes will follow the pressure cycling.
According to yet another aspect of the invention, a determination of the size of the asphaltene precipitates is found by using the Stokes equation which relates the particle size to the particle velocity, the viscosity of the oil, and the densities of the particles and oil. It has been found that the optical density of a precipitating sample at a given pressure will decrease over time, as the asphaltenes precipitate out. The velocity of the particles may therefore be measured by tracking a decline in the optical density of a precipitating sample over a period of time; e.g., by knowing the sample cell height, and by finding the amount of time it takes for the optical density to decline to some percentage (e.g., 1/e) of the difference between a maximum optical density and a baseline measurement.
All methods of the invention may be carried out both uphole and downhole, and if downhole, using a borehole tool or using permanently located optical cells. The Stokes equation measurement for finding the particle size, however, is most suited to uphole measurement.
Additional objects and advantages of the invention will become apparent to those skilled in the art upon reference to the detailed description taken in conjunction with the provided figures.